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PubMed1993

Crucial effects of weightlessness on human orientation.

This contribution examines the consequences of two remarkable experiences of subjects in weightlessness, 1) the missing of sensations of trunk tilt and of the respective concomitant reflexes when the head is tilted with respect to the trunk, and 2) the persistence of a perception of "up" and "down," that is, of the polarity of the subjective vertical (SV) in the absence of, as well as in contradiction to, visual cues. The first disproves that the necessary head-to-trunk coordinate transformation be achieved by adding representations of the respective angles gained by utricles and neck receptors, but corroborates an extant model of cross-multiplication of utricular, saccular, and neck receptor components. The second indicates the existence of force-independent components in the determination of the SV. Although the number of subjects is still small and experimental conditions are not as homogeneous as desired, measurements and/or reports on the ground, in parabolic, and in space flight point to the decisive role of the saccular z-bias, that is, of a difference of the mean resting discharges of saccular units polarized in the rostrad and the caudad (+/- z-) direction.

Humans↗

PubMed2004

Copper deficiency myelopathy produces a clinical picture like subacute combined degeneration.

BACKGROUND: Copper deficiency in ruminants is known to cause an ataxic myelopathy. Copper deficiency as a cause of progressive myelopathy in adults is underrecognized. OBJECTIVE: To describe the clinical, biochemical, electrophysiologic, and imaging characteristics in 13 patients with myelopathy associated with copper deficiency. METHODS: The records of patients with a copper deficiency-associated myelopathy were reviewed. Clinical characteristics, laboratory investigations, and responses to therapeutic intervention were summarized. RESULTS: Thirteen such patients were found, 11 of them in a 15-month period. All patients presented with prominent gait difficulty, reflecting a sensory ataxia due to dorsal column dysfunction and lower limb spasticity. All patients had polyneuropathy. A high or high-normal serum zinc level was seen in 7 of the 11 patients for whom this information was available. Somatosensory evoked potential studies done in eight patients showed impaired conduction in central proprioceptive pathways. Dorsal column signal change on spine MRI was present in three patients. An initial clue to the diagnosis was a very low ceruloplasmin level; further tests of copper metabolism excluded Wilson disease. The cause remained unexplained in most patients. Oral copper supplementation restored normal or near-normal copper levels in 7 of the 12 patients in whom adequate follow-up data were available; parenteral supplementation restored normal level in 3 further patients. Copper supplementation prevented further neurologic deterioration, but the degree of actual improvement was variable. CONCLUSIONS: Unrecognized copper deficiency appears to be a common cause of idiopathic myelopathy in adults. The clinical picture bears striking similarities to the syndrome of subacute combined degeneration associated with vitamin B12 deficiency. Early recognition and copper supplementation may prevent neurologic deterioration.

Adult↗

PubMedDate not supplied

[The acquisition in dogs of temporal regulation by differentiated reinforcement of the response duration using an exteroceptive component and the pharmacological modulation of this reflex (diazepam, clozapine)].

Transformation of expectation phenomenon into the phenomenon of temporal regulation is usually achieved by the suppression of preliminary factors or by means of their physical modification. Our studies show that such transformation can be obtained in dogs using Kupalov paradigm with the presentation of additional stimuli. These stimuli strictly identical, from the physical point of view, to the signals which interrupt expectation are randomly introduced into the temporal limit. Absence of the reinforcement in response to the additional stimulus impels the animal to include temporal regulation in its behaviour, and an additional negative discriminative stimulus promotes an expression of active character of inhibition. These circumstances make our pattern closer to DRRD (differential reinforcement of response duration). In order to evaluate the merits of this procedure the influence was studied of anxiolytic (diazepam) and neuroleptic (closepine) on the stabilized reaction of experimental animals. The increase of responses duration by closepine and their shortening by diazepam as well as the influence of these pharmacological substances on the frequency of responses in dependence of dose, confirm the results of the previous studies of DRRD and DRL (differential reinforcement of low rate of responses) and prove differential sensitivity of our procedure to pharmacological substances.

Animals↗

PubMed2005

The central termination of sensory fibers from nerves to the gastrocnemius muscle of the rat.

Peripheral nerves innervating muscles have sensory fibers that relay information into the CNS information about proprioception, pain, and the metabolic state of the muscle. The present study shows the primary afferent projections into the spinal cord of the nerves innervating the gastrocnemius muscle of the rat using the transganglionic transport of a cocktail of horseradish peroxidase (HRP) conjugated to cholera toxin and wheat germ agglutinin; these markers have been shown to label large and small fibers, respectively. A dense projection into lamina I of the lumbar dorsal horn and a more moderate projection into lamina V were seen. Moreover, dense reaction product was found in the most medial aspect of lamina II, especially lamina II inner part, and less in lamina III and IV of levels L3-L5. Lamina VI had dense reaction product from the rostral sacral levels of the spinal cord that continued into Clarke's column at rostral lumbar levels. The nucleus gracilis also was labeled. Other nerves emerging from the popliteal fossa, including the tibial, peroneal, and sural nerves, also were injected with the HRP cocktail and their projections compared with those from the gastrocnemius muscle. Projections from the gastrocnemius muscle only partially overlapped with those from the tibial nerve, from which the nerves to the gastrocnemius muscle branch. However, the topology of projections from these nerves to laminae II-IV of the dorsal horn differed from that of the nerves of the gastrocnemius muscle, suggesting there was little spread to other nerves in the popliteal fossa. It was also noted that large labeled processes, presumably dendrites of retrogradely labeled motoneurons, entered the dorsal horn. These data provide information on the central projections of both the large and small fibers innervating the gastrocnemius muscle, and may aid in determining the circuitry utilized in the exercise pressor reflex as well as muscle pain.

Afferent Pathways↗

PubMed2000

Mechanical cutaneous stimulation alters Ia presynaptic inhibition in human wrist extensor muscles: a single motor unit study.

Reflex responses were evoked by radial nerve stimulation in 25 single motor units in the extensor carpi radialis muscles of seven subjects during voluntary isometric wrist extension. The responses consisted of narrow peaks in the post-stimulus time histograms with latencies compatible with monosynaptic activation. When the skin of the palm and finger tips was continuously swept using a soft rotating brush, the purely monosynaptic components of the motor unit responses, as assessed from the contents of the first two 0.25 ms bins of the peak, were found to increase. This increase did not affect the motoneurone net excitatory drive, as assessed by measuring the mean duration of the inter-spike intervals. The cutaneous inputs activated by the brush may have reduced the tonic presynaptic inhibition exerted on the Ia afferents homonymous to the extensor motor units tested. To further investigate whether Ia presynaptic inhibition was involved, the responses of the extensor motor units were conditioned by stimulating the median nerve 20 ms earlier, using a protocol which is known to induce Ia extensor presynaptic inhibition originating from flexor Ia afferents. The median nerve stimulation did not affect the motoneurone excitatory drive, but led to a decrease in the responses of the extensor motor units to the radial nerve stimulation, especially in the purely monosynaptic components. This decrease was consistent with the Ia presynaptic inhibition known to occur under these stimulation conditions. The cutaneous inputs activated by the brush were found to reduce the Ia presynaptic inhibition generated by the median nerve stimulation, without affecting the distribution of the Ia presynaptic inhibition among the various types of motor units tested. The present data suggest that cutaneous inputs from the palm and finger tips may relieve the Ia presynaptic inhibition exerted on the wrist extensor motor nuclei, and thus enhance the proprioceptive assistance to fit the specific requirements of the ongoing motor task.

Adult↗

PubMed1994

Corrective responses to loss of ground support during walking. I. Intact cats.

1. In the cat step cycle the electromyographic (EMG) activity in ankle extensor muscles commences approximately 70 ms before foot contact. There is a sharp peak between 10 and 25 ms after contact and the EMG then declines for the remainder of the stance phase. It has been posited that the abrupt transition in EMG after contact is the consequence of reflexes elicited by the large barrage of afferent input that signals foot touchdown. However, it is also possible that the basic profile might be generated within the CNS, with little modification by afferent input. 2. These ideas were tested in 11 normal cats. We compared EMG responses and hindlimb kinematics in steps with normal ground support and steps in which an actuator-controlled trap door unexpectedly opened, withdrawing ground support just before foot contact. 3. In the absence of ground support the transition in EMG activity was still present. The averaged EMG pattern was similar for at least 30 ms after the foot passed through the plane of the floor. We conclude that the basic extensor activation profile in this part of the cycle is generated centrally and is not substantially altered by afferent input. 4. Between 35 and 200 ms after contact the stance phase was aborted and the foot was lifted smartly out of the hole. This reaction varied both in latency and kinematic detail, suggesting a fairly complex corrective response.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗

PubMed1992

Influence of gravitoinertial force level on vestibular and visual velocity storage in yaw and pitch.

Velocity storage is an important aspect of sensory-motor control of body orientation. The effective decay rate and three-dimensional organization of velocity storage are dependent upon body orientation relative to gravity and also are influenced by gravitoinertial force (G) level. Several of the inputs to velocity storage including otolithic, somatosensory, proprioceptive, and possibly motor are highly dependent on G level. To see whether the G dependency of velocity storage is related to changes in the effective coupling of individual sensory inputs to the velocity storage mechanism or to alterations in the time constant of velocity storage per se, we have studied horizontal vestibular nystagmus, horizontal optokinetic after nystagmus (OKAN) and vertical vestibular nystagmus as a function of force level. Horizontal OKAN and vestibular nystagmus both showed no effect of G level on their initial or peak slow phase velocities but their decay rates were quicker in 0G and 1.8G than in 1G. Vertical vestibular nystagmus also showed no effect of G level on peak velocity but decayed quicker in 0G relative to 1G. These-findings indicate that the intrinsic decay rate of a common velocity storage mechanism is affected by the magnitude of G. A negligible amount of slow phase eye velocity was observed in planes outside the planes of stimulation, thus short-term changes in G across multiple body axes can change velocity storage, but the change is restricted to the axis common to the rotary stimulus and the G vector.

Aerospace Medicine↗

PubMed1985

[Clinical neurophysiology in the evaluation and physiopathology of Parkinson's disease].

Clinical Neurophysiology brings about 2 major contributions in the study of Parkinson's disease: on the one hand, it makes it possible to measure the motor troubles; on the other, it enables their pathophysiological analysis. The 3 classical signs must be studied separately. Tremor can easily be recorded by electromyography. Moreover, its parkinsonian nature can be specified by studying the resetting of EMG bursts following electrical stimulation of the motor nerve. A pace maker has been demonstrated in the thalamus from where rhythmic messages are first sent to motor cortex and thereafter reverberated to spinal motoneurons. Rigidity can be assessed by sophisticated but not generalized methods. It is easier to evaluate it by long-loop responses evoked by proprioceptive or exteroceptive stimulations. These responses reflect activity in pathways relaying in supraspinal structures. Contrary to spasticity, rigidity is not basically due to dysfunctions in segmentary spinal circuits. It is more likely that it depends on hyperactive and hyperexcitable long loop pathways. This hypothesis is in agreement with well established facts showing that parkinsonian hypertonia vanishes after dorsal root section. Akinesia is complex semeiologically. It is made of various components some of which can be measured. Reaction times and movement times provide interesting data which however are not strictly correlated with the motor handicap. Motor programmes are assembled in normal delays but they are not "called upon" correctly, reflecting a disturbance in the motor planning. A lack of "energetization" of the motor cortex and the pyramidal tract is likely. A functional disconnection between the motor program/plan side and the execution side can be hypothesized to explain the 3 major signs; on the one hand, neural messages are not correctly transferred to the pyramidal system, on the other, spinoencephalospinal loops on the execution side become more active as they escape from the control normally exerted by the plan/program side where basal ganglia play a prominent role.

Animals↗

PubMed1985

Canal-neck interaction in vestibular neurons of the cat's cerebral cortex.

Interaction of semicircular canal and neck proprioceptive inputs was studied in the cerebral cortex of awake, intact cats. Neuronal responses were recorded extracellularly in the anterior suprasylvian gyrus of the left hemisphere. Stimulations consisted of horizontal rotations in the dark applied as sinusoids or position ramps. There were three stimulus conditions: (1) Pure canal stimulation; rotation of whole body. (2) Pure neck stimulation; rotation of trunk about stationary head. (3) Canal-neck interaction; rotation of head about stationary trunk. We recorded 105 neurons with either Type I or Type II canal response. These showed often pronounced non-linearities such as a clear firing increase upon rotation in the "on-direction" and hardly any decrease in the opposite direction. The responses reflected mostly angular velocity, but angular position signals were also obtained. In 79 neurons, either Type I or Type II neck responses were obtained. They coded either angular velocity, velocity plus position, or position. Canal-neck convergence was found in 67 of 88 neurons tested. In the majority of neurons, interaction was "antagonistic" in the sense that the canal and neck responses tended to cancel each other during rotation of the head about the stationary trunk. These neurons could signal trunk rotation in space rather than head in space or head relative to trunk. Most of the remaining neurons showed a "synergistic" interaction such that the response upon head rotation was enhanced as compared to whole body or trunk rotation. These neurons might be involved in the dual task of monitoring head rotation in space and relative to trunk. Interaction was compatible with linear summation of canal and neck inputs in 70% of the neurons. In part of these, however, the assumption had to be made that the interaction had taken place already at some stage prior to the cortical neurons investigated. The response characteristics of cortical canal neurons are discussed in comparison to vestibular nuclear neurons. Furthermore, parallels are drawn between the observed canal-neck interactions in the cortical neurons and (i) interactions of canal and neck dependent postural reflexes in the decerebrate cat, and (ii) interactions of canal and neck induced turning sensations in man.

Animals↗

PubMed2006

Balance control in peripheral neuropathy: are patients equally unstable under static and dynamic conditions?

The aim of this investigation was to assess the potentially different effects of impaired proprioceptive input in balance control under static and dynamic conditions in neuropathic patients. We recruited 20 normal subjects and 27 patients affected by neuropathies known to affect to a different extent large and medium size afferent fibres. The patients comprised 5 with Charcot-Marie-Tooth disease type 1A (CMT1A), 8 with CMT type 2 (CMT2) and 14 with Diabetes polyneuropathy (Diabetics). Measurement of balance during quiet stance on a stabilometric platform showed that sway area (SA) was larger in the CMT2 and Diabetics than normal subjects or in CMT1A, under both eyes open and closed conditions. The estimated conduction velocity (CV) of the group II afferent fibres was lower in CMT2 and Diabetics than in normal subjects and CMT1A. Across all patients, SA increased as a function of the slowing of group II CV. During a dynamic balance task the head A-P displacement was only slightly increased in the patient groups with respect to normals, despite the increased delay at which the head followed displacement of the feet. The unpredictably good performance of all patient groups under dynamic condition, which was at variance with their imbalance during quiet stance, may indicate that these patients learnt to exploit anticipatory postural strategies.

Adult↗

PubMed2002

Altered triggering of a prepared movement by a startling stimulus.

An experiment is reported that investigated the effects of an auditory startling stimulus on a compound movement task. Previous findings have shown that, in a targeting task, a secondary movement can be initiated based on the proprioceptive information provided by a primary movement. Studies involving the presentation of a startling stimulus have shown that in reaction time (RT) tasks, prepared ballistic movements could be released early when participants are startled. In the present study we sought to determine whether the secondary component in an ongoing movement task, once prepared, could also be triggered by a startling stimulus. Participants performed a slow active elbow extension (22 degrees /s), opening their hand when the arm passed 55 degrees of extension from the starting point. An unexpected 124 dB startle stimulus was presented 5, 25, or 45 degrees into the movement. Findings showed that, when participants were startled, the secondary component was triggered despite incongruent kinesthetic information. However, this only occurred when the startle was presented late in the primary movement. This suggests that the secondary movement was not prepared prior to task initiation, but was "loaded" into lower brain structures at some point during the movement in preparation to be triggered by the CNS. This occurred late in the movement sequence, but >/=400 ms prior to reaching the target. These findings indicate that, in addition to ballistic RT tasks, a startle can be used to probe response preparation in ongoing compound movement tasks.

Adult↗

PubMed2002

Modulations in corticomotor excitability during passive upper-limb movement: is there a cortical influence?

Modulations in the excitability of corticomotor pathways to forearm musculature have previously been demonstrated during passive wrist movement [Brain Res. 900 (2001) 282]. Investigations were conducted to determine the level of the neuroaxis at which these modulations arise, and to establish the influence of proprioceptive task constraints on pathway excitability. Forearm motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were examined during passive wrist movement while subjects maintained a low-level muscle activation, thus stabilising the excitability of the motoneuron pool. Modulations in response amplitude during movement were evident in both forearm flexor and extensor muscles. The pattern of modulation generally mirrored that seen in quiescent musculature during movement, with responses potentiated during the phases where the muscle was in a shortened position. Variations in MEP amplitude were not detected while the wrist was constrained statically at various joint angles. This suggests a dynamic influence of movement, most likely mediated by spindle receptors, arising at a supraspinal level. We also investigated the influence of a kinesthetic tracking task on corticomotor excitability during passive movement of the wrist joint. MEPs were recorded from the target driven limb while the contralateral limb was stationary, while the contralateral limb actively tracked the movements of the target limb, and while the contralateral limb moved actively in time with a metronome. The results revealed no differences in MEP characteristics in the driven limb between the three conditions. Placing the movement elicited afferent information in an active movement context does not appear to enhance the modulations in cortical excitability.

Adult↗

PubMed1997

[Changes in posture in whiplash evaluated by static posturography].

Analyzing vestibular-spinal reflex by recording changes in the pressure center over time, static posturography provides precise, repeatable information regarding a person's posture and subsequent postural strategy. Moreover, using the detected postural pattern, this can presage likely genesis of problems in equilibrium. The purpose of the present work was to evaluate posturographic findings in subjects with chronic or persistent whiplash (WL)-induced disequilibrium in order to determine any clear postural pattern able to indicate the likely genesis of the disorder. Fifty young subjects were selected for the study: 28 (6 females) affected with equilibrium problems resulting from WL-type cervical strain and 22 (5 females) normal, age compatible controls (C) (mean age of the groups: WL = 37 +/- 14.3; C = 32.4 +/- 6.7; p = n.s.). The results obtained were statistically processed with a personal computer and specific software (Windows-SPSS). In the analysis of the means (T-test and ANOVA analysis of variance) p < 0.05 was considered significant. In the WL group there was an increase in the antero-posterior axis oscillation frequency as compared to the control group. This is indicated by the FFTY parameter in tests performed both with eyes open and closed. Likewise, in both tests, there was a decrease in the LFS value, a parameter correlating length with surface. The surface was increased in the WL group as compared to group C although this increase was significant only when the test was performed with eyes closed. In addition, the average velocity and standard deviation of velocity increased in the WL group but this was only statistically significant when the eyes were open. The results obtained in the present study show that whiplash victims show an increase in antero-posterior oscillation frequency at the pressure center, as found in the open and closed-eye tests. This suggests that in WL subjects there is an evident disturbance of the fine postural system, leading to an increase in the surface parameters, tracing length and a decrease in LFS function. The onset of this partial postural decompensation can be seen in: a) decrease in fine static postural control which is reflected in an increase in body surface oscillation; b) an increase in energy consumption to maintain orthostatism as revealed in the decreased LFS. In conclusion, it appears reasonable to assume that cervical proprioceptive alterations play a preeminent role in the genesis of whiplash-induced chronic postural instability. This would result in an attempt to vary the physiology from an ankle to a hip strategy; incomplete manifestation of this new posture would cause the feeling of instability mentioned by the patients and documented by posturography.

Adult↗

PubMed1994

Transmission in a locomotor-related group Ib pathway from hindlimb extensor muscles in the cat.

It has been previously shown that phasic stimulation of group I afferents from ankle and knee extensor muscles may entrain and/or reset the intrinsic locomotor rhythm; these afferents are thus acting on motoneurones through the spinal rhythm generators. It was also concluded that the major part of these effects originates from Golgi tendon organ Ib afferents. Transmission in this pathway to lumbar motoneurones has now been investigated during fictive locomotion in spinal cats injected with nialamide and L-DOPA, and in decerebrate cats with stimulation of the mesencephalic locomotor region. In spinal cats injected with nialamide and L-DOPA, it was possible to evoke long-latency, long-lasting reflexes upon stimulation of high threshold afferents before spontaneous fictive locomotion commenced. During that period, stimulation of ankle and knee extensor group I afferents evoked oligosynaptic excitation of extensor motoneurones, rather than the "classical" Ib inhibition. Furthermore, a premotoneuronal convergence (spatial facilitation) between this group I excitation and the crossed extensor reflex was established. During fictive locomotion, in both preparations, the transmissions in these groups I pathway was phasically modulated within the step cycle. During the flexor phase, the group I input cut the depolarised (active) phase in flexor motoneurones and evoked EPSPs in extensor motoneurones; during the extensor phase the group I input evoked smaller EPSPs in extensor motoneurones and had virtually no effect on flexor motoneurones. The above results suggest that the group I input from extensor muscles is transmitted through the spinal rhythm generator and more particularly, through the extensor "half-centre". The locomotor-related group I excitation had a central latency of 3.5-4.0 ms. The excitation from ankle extensors to ankle extensors remained after a spinal transection at the caudal part of L6 segment; the interneurones must therefore be located in the L7 and S1 spinal segments. Candidate interneurones for mediating these actions were recorded extracellularly in lamina VII of the 7th lumbar segment. Responses to different peripheral nerve stimulation (high threshold afferents and group I afferents bilaterally) were in concordance with the convergence studies in motoneurones. The interneurones were rhythmically active in the appropriate phases of the fictive locomotor cycle, as predicted by their response patterns. The synaptic input to, and the projection of these candidate interneurones must be fully identified before their possible role as components of the spinal locomotor network can be evaluated.

Animals↗

PubMed2001

Corticospinal excitation of presumed cervical propriospinal neurones and its reversal to inhibition in humans.

1. This study addresses whether in human subjects indirect corticospinal excitation of upper limb motoneurones (MNs) relayed through presumed cervical propriospinal neurones (PNs) is paralleled by corticospinal activation of inhibitory projections to these premotoneurones. 2. The responses to transcranial magnetic stimulation (TMS), whether assessed as the compound motor-evoked potential (MEP) or the peak of corticospinal excitation elicited in the post-stimulus time histograms (PSTHs) of single motor units, were conditioned by weak volleys to musculo-cutaneous, ulnar and superficial radial nerves. 3. Afferent volleys, which hardly modified the H reflex, significantly facilitated the corticospinal response produced by weak TMS. In PSTHs, the central delay of the peripheral facilitation of the peak of corticospinal excitation in MNs located at either end of the cervical enlargement was longer the more caudal the MN pool, suggesting an interaction in premotoneurones located rostral to the tested MNs. 4. Small increases in the strength of TMS (approximately 2--5 % of the maximal stimulator output) caused the facilitation to disappear and then to be reversed to inhibition. The facilitatory and inhibitory effects had the same latencies and spared the initial 0.5--1 ms of the corticospinal excitatory response. Both effects were more readily demonstrable when there was a co-contraction of the target muscle and the muscle innervated by nerve used for the conditioning stimulus. 5. The above features suggest that the inhibition resulted from disfacilitation due to suppression of corticospinal excitation passing through the presumed premotoneuronal relay. The reversal of the facilitation to inhibition by stronger corticospinal volleys is consistent with a well-developed system of 'feedback inhibitory interneurones' activated by corticospinal and afferent inputs inhibiting the presumed propriospinal excitatory premotoneurones. 6. It is argued that these findings might explain why simply stimulating the pyramidal tract or the motor cortex would fail to demonstrate this indirect corticospinal projection in the macaque monkey and in humans.

Adult↗

PubMed2006

Modelling spinal circuitry involved in locomotor pattern generation: insights from deletions during fictive locomotion.

The mammalian spinal cord contains a locomotor central pattern generator (CPG) that can produce alternating rhythmic activity of flexor and extensor motoneurones in the absence of rhythmic input and proprioceptive feedback. During such fictive locomotor activity in decerebrate cats, spontaneous omissions of activity occur simultaneously in multiple agonist motoneurone pools for a number of cycles. During these 'deletions', antagonist motoneurone pools usually become tonically active but may also continue to be rhythmic. The rhythmic activity that re-emerges following a deletion is often not phase shifted. This suggests that some neuronal mechanism can maintain the locomotor period when motoneurone activity fails. To account for these observations, a simplified computational model of the spinal circuitry has been developed in which the locomotor CPG consists of two levels: a half-centre rhythm generator (RG) and a pattern formation (PF) network, with reciprocal inhibitory interactions between antagonist neural populations at each level. The model represents a network of interacting neural populations with single interneurones and motoneurones described in the Hodgkin-Huxley style. The model reproduces the range of locomotor periods and phase durations observed during real locomotion in adult cats and permits independent control of the level of motoneurone activity and of step cycle timing. By altering the excitability of neural populations within the PF network, the model can reproduce deletions in which motoneurone activity fails but the phase of locomotor oscillations is maintained. The model also suggests criteria for the functional identification of spinal interneurones involved in the mammalian locomotor pattern generation.

Action Potentials↗

PubMed2004

Probing the human vestibular system with galvanic stimulation.

Galvanic vestibular stimulation (GVS) is a simple, safe, and specific way to elicit vestibular reflexes. Yet, despite a long history, it has only recently found popularity as a research tool and is rarely used clinically. The obstacle to advancing and exploiting GVS is that we cannot interpret the evoked responses with certainty because we do not understand how the stimulus acts as an input to the system. This paper examines the electrophysiology and anatomy of the vestibular organs and the effects of GVS on human balance control and develops a model that explains the observed balance responses. These responses are large and highly organized over all body segments and adapt to postural and balance requirements. To achieve this, neurons in the vestibular nuclei receive convergent signals from all vestibular receptors and somatosensory and cortical inputs. GVS sway responses are affected by other sources of information about balance but can appear as the sum of otolithic and semicircular canal responses. Electrophysiological studies showing similar activation of primary afferents from the otolith organs and canals and their convergence in the vestibular nuclei support this. On the basis of the morphology of the cristae and the alignment of the semicircular canals in the skull, rotational vectors calculated for every mode of GVS agree with the observed sway. However, vector summation of signals from all utricular afferents does not explain the observed sway. Thus we propose the hypothesis that the otolithic component of the balance response originates from only the pars medialis of the utricular macula.

Afferent Pathways↗

PubMed1995

Central connections of sensory neurones from a hair plate proprioceptor in the thoraco-coxal joint of the locust.

The hair plate proprioceptors at the thoraco-coxal joint of insect limbs provide information about the movements of the most basal joint of the legs. The ventral coxal hair plate of a middle leg consists of group of 10-15 long hairs (70 microns) and 20-30 short hairs (30 microns). The long hairs are deflected by the trochantin as the leg is swung forward during the swing phase of walking, and their sensory neurones respond phasically during an imposed deflection and tonically if the deflection is maintained. Selective stimulation of the long hairs elicits a resistance reflex that rotates the coxa posteriorly and is similar to that occurring at the transition from the swing to the stance phase of walking. The motor neurones innervating the posterior rotator and adductor coxae muscles are excited, and those to the antagonistic anterior rotator muscle are inhibited. By contrast, selective stimulation of the short hairs leads only to a weak inhibition of the anterior rotator. The excitatory effects of the long hairs are mediated, in part, by direct connections between their sensory neurones and particular motor neurones. A spike in a sensory neurone elicits a short-latency depolarising postsynaptic potential (PSP) in posterior rotator and adductor motor neurones whose amplitude is enhanced by hyperpolarising current injected into the motor neurone. When the calcium in the saline is replaced with magnesium, the amplitude of the PSP is reduced gradually, and not abruptly as would be expected if an interneurone were interposed in the pathway. Several sensory neurones from long hairs converge to excite an individual motor neurone, evoking spikes in some motor neurones. The projections of the sensory neurones overlap with some of the branches of the motor neurones in the lateral association centre of the neuropile. It is suggested that these pathways would limit the extent of the swing phase of walking and contribute to the switch to the stance phase in a negative feedback loop that relieves the excitation of the hairs by rotating the coxa backwards.

Animals↗